The invention relates to a device for detecting and preprocessing weights acting on a vehicle seat.
In the course of introducing airbags as standard for the passenger seat in motor vehicles there arises the problem of detecting the occupancy of the seat, so that the passenger airbag is released for firing only in those cases in which the seat is occupied. Firing of the passenger airbag when the passenger seat is not occupied not only has the consequence that an additional increase in pressure in the vehicle is unnecessarily caused in the event of an accident by the firing of the passenger airbag, but also that, due to the integration of the passenger airbag into the dashboard, considerable repair measures are necessary in the passenger compartment of the vehicle following firing of the airbag.
Various problems which have been observed when airbags are fired with small children or youngsters traveling on the passenger seat have ultimately led to the requirement that, in the event of an accident, an airbag must be operated in a way which is adapted to the person sitting on the vehicle seat.
This gives rise to the problem of determining the necessary data for firing the airbag in a way appropriate for requirements.
One possible reference point is to detect the weight of the respective vehicle occupant, the size and physical robustness of the occupant then being concluded on the basis of a weight determination.
U.S. Pat. No. 5,739,757 proposes using sensors incorporated in the seat cushion of a vehicle seat to determine the weight of the vehicle occupant and to fire the airbag in accident situations only in the case in which a lower weight limit is exceeded. In addition, the position of the occupant on the seat is determined, so that the airbag can be switched off if the occupant is too close to the airbag for safe firing. Recommended in this case as sensors are ferromagnetic elements which change their magnetic permeability under the horizontal tensile loading occurring here.
It is a disadvantage of this solution that changes in the spring constant of the seat suspension (aging, use) lead to an additional zero drift of the weighing signal of the sensor elements. In addition, considerable measuring errors occur when the vehicle accelerates or decelerates, and have to be corrected in a complex way. An adverse effect of these measuring errors is that they correspond to interference signals with a long period/low frequency. As a result, a more time-consuming correction has to be performed, making the measuring system intrinsically slow to respond. In addition, when the vehicle is inclined, a great measuring error is obtained, and can only be corrected with considerable effort.
DE 38 09 074 A1 proposes disposing weight sensors in the region of the seat rail of the vehicle seat to determine the sitting position or position of the center of gravity of the vehicle occupant and to decide according to the sitting position whether the airbag is partly or fully fired and deployed in an accident situation. Recommended quite generally here are inductive sensors, with no recommendation being given for the actual construction of the sensors.
A particular disadvantage of this solution is that the seat rails have to be disposed higher above the floor of the vehicle than would otherwise be necessary without the sensors. This, however, impairs the headroom for the occupants of the vehicle.
In weighing technology there are many different known load cells which have their weighing signal preprocessed and displayed.
The load cells comprise a force transducer and a sensor element, which supplies a measuring signal when the force transducer is subjected to loading.
Known force transducers are in many cases based on the deformation being detected as a surface expansion. Typical of these are strain-gage transducers, in which the surface expansion occurring when there is deformation is converted into a change in electrical resistance.
These strain-gage transducers achieve very high degrees of accuracy, but can be protected against ambient influences only with great effort, since the strain-sensitive sensor elements are located directly on the elastically deforming part and covering of the sensors has the concomitant effect of force bypasses, which have direct adverse effects on the properties of the transducer.
In the case of capacitively operating load cells, elastic deformation of the deformable part causes a change in the spacing between two capacitor plates, and consequently in the capacitance of the latter, whereas in the case of the inductively operating force transducers described at the beginning the spatial distribution of an alternating magnetic field is changed by the deformation, which leads to a change in the inductance of a pickup coil of the sensor element.
DE 44 20 691 C1 discloses inductively operating load cells which are designed specifically for applications in weighing technology. These prove to be unsuitable under extreme ambient conditions, in particular such as occur for example in a motor vehicle, since they are too susceptible to interference.
In terms of measuring technology, the vehicle interior is extremely problematical, since not only do interference signals of the engine ignition (electromagnetic fields with high field strengths) occur to a considerable extent and have to be eliminated in signaling terms, but it is also necessary with respect to the functional capability of sensors that are to be used to take into account extreme vibrations, temperature conditions in a range from, for example, −40° C. to +85° C., rapid temperature changes and also atmospheric humidity ranging right up to the dew point.
Furthermore, account must be taken of the fact that not only the weights of the occupants act on the seat but also, when the vehicle accelerates and, in particular, decelerates suddenly, forces which are many times greater, not to mention the forces occurring in an accident situation, which likewise must not lead to malfunctioning of the sensors.
As well as this there are the various adjusting possibilities for a vehicle seat which are provided in modern motor vehicles and may only influence the measuring result within defined limits.